This application claims the priority of German Patent Document DE 10 2005 029 137.6-13, filed Jun. 23, 2005, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a control device and a method for diagnosing a charge cycle valve stroke adjusting system of an internal-combustion engine having at least one exhaust gas sensor which supplies an output signal continuously varying with an exhaust gas constituent.
Such a method and such a control device are disclosed in German Patent Document DE 197 36 064. Moreover, details for a mechanical implementation of a stroke of a charge cycle valve, which can be changed between a minimal and a maximal value, are contained, for example, in German Patent Document DE 196 06 054 C2 (compare particularly FIG. 2 there). A continuous lambda sensor, which can be used as an example for the initially mentioned exhaust gas sensor, is explained, for example, in the “Automotive Handbook”, 23rd Edition (ISBN 3-528-03876-4) under the name “Broadband Sensor” on Page 524.
In internal-combustion engines with an adjustable valve stroke, for a given set of operating conditions (for example, rotational speed, temperature, throttle valve opening angle, etc.), the charge of the cylinder/combustion chamber depends on the actually occurring valve stroke. In the case of an operating point plane defined by values of the combustion chamber charge and the rotational speed, when the charges and rotational speeds are low, a small valve stroke is adjusted, which (due in part to the connected higher inflow velocity) improves the mixture preparation in the combustion chamber. If, because of a defect the actual value of the valve stroke does not correspond to its desired value, the air charge also does not correspond to its desired value, which impairs the exhaust gas quality, among other effects. Because of legal requirements, such an exhaust-gas-relevant defect has to be detected by means of on-board devices when used in motor vehicles.
The initially mentioned German Patent Document DE 197 36 064 discloses that such defects can be derived from the signal of continuous lambda sensors. It is suggested there that, in the case of an internal-combustion engine with two continuous lambda sensors, which are each arranged in the exhaust line of a subset of the cylinders of the internal-combustion engine, the difference between the signals of the two sensors is analyzed. If this difference exceeds a predefined threshold value, it is evaluated to be a defect. If the defect occurs in a time relation with a change of the valve stroke, this defect is assigned to the valve stroke adjusting system.
In the case of multi-cylinder internal-combustion engines, various defects may occur in the valve stroke adjusting system. Thus, the valve stroke adjustment of an entire cylinder group (cylinder bank) may operate defectively. However, it is also conceivable that only a single cylinder is affected.
In the former case, a large difference between the lambda sensor signals will occur; while, in the second case, only a smaller difference will occur. A defect detection threshold value therefore has to be so low that it is also exceeded in the event of a defect of an individual cylinder. A low threshold value increases the sensitivity of the diagnosis and thus also the probability that differences occurring in an operation without defects are erroneously detected as defects.
In view of this background, it is an object of the invention to provide a method and apparatus which permit a detection of defects of a valve stroke adjusting system with an increased reliability, and without losses of sensitivity of the detection of actual defects.
This and other objects and advantages are achieved by the method and apparatus according to the invention, in which a measurement of a spread width of the output signal is determined and compared with a reference value. Defects of the valve stroke adjusting system are detected on the basis of the comparison.
In addition to achieving improved reliability when the error detection sensitivity is high, the method according to the invention also has the advantage that it can be used in internal-combustion engines that have only one continuous exhaust gas sensor. This advantage is the result of the fact that defects of the valve stroke adjustment are illustrated by the spread width of the output signal of the exhaust gas sensor.
With a view to further developments of the method, it is preferable that the reference value be dependent on operating parameters of the internal-combustion engine. This development further increases the system's reliability because operating-point-dependent influences on the spread width can be taken into account during the determination of the reference value. For operating points at which comparatively large spread widths occur even when the valve stroke adjusting system has no defect, the reference value is adapted correspondingly.
It is also preferred that, in the case of a first desired value of the valve stroke, the first measurement of a spread width of an output signal of the exhaust gas sensor is defined as the reference value for a second measurement of the spread width of the output signal of the same exhaust gas sensor in the case of a second desired value of the valve stroke. Such before/after comparison also has the advantage that it can be carried out by means of only one exhaust gas sensor. A change of the spread width, which is in a time relation with an output of a changed desired valve stroke value, represents a reliable indication of a defect of the valve stroke adjustment.
It is also preferred that, in the case of an internal-combustion engine with several groups of cylinders and exhaust gas sensors for each cylinder group, a third measurement of a spread width of an output signal of a first exhaust gas sensor of a first group of cylinders is defined as the reference value for a fourth measurement of a spread width of an output signal of a second exhaust gas sensor of a second group of cylinders.
Operating-point-dependent influences change the output signals of both the first exhaust gas sensor and the second exhaust gas sensor. By forming the reference value as a function of the spread width of one of the exhaust gas sensors, operating-point-dependent influences are therefore automatically taken into account. Another advantage lies in the fact that a drift of such influences is also automatically taken into account, because the drift has an effect on the output signals of both exhaust gas sensors.
In another preferred embodiment, the difference between the measurement of the spread width and the reference value is formed and compared with a threshold value if it exceeds the threshold value, a defect is determined to exist. This embodiment permits the definition of a tolerable deviation between the reference value and the measurement for the spread width, which has a positive influence on the reliability of the diagnosis.
It is also preferred that a range of possible values of an output signal of an exhaust gas sensor is divided into different classes of a defined width. A number of classes is then determined in which the output signal or a filtered output signal extends between two successive local extremes (of the output signal). A sum of a predetermined number of detected figures is formed and is used as a measurement for the spread width.
Practical tests have shown that this type of diagnosis, which is also called “rain flow classification”, supplies particularly reliable results.
In addition, it is preferred that the filtered output signal is generated by high-pass filtering of the output signal, which has the advantage that a control fluctuation (which may occur in the output signal of the exhaust gas sensor) is filtered out before the analysis. Such a control fluctuation is created by the time delay which occurs between the formation of the mixture before the combustion, and the point in time at which the exhaust gas resulting from the combustion is recorded at the installation site of the exhaust gas sensor. Such a control fluctuation typically has a frequency on the order of 2 Hz, while a valve stroke defect for an individual cylinder of a four-stroke engine (2 crankshaft rotations per working cycle of a cylinder), at a rotational speed of 3,600 min−1, assumed as an example, with a frequency of 3,600/50/2 Hz=30 Hz is exhibited in the output signal of an exhaust gas sensor.
In still a further embodiment of the invention, an average value of the output signal is formed, and an integral of absolute amounts of deviations of the output signal from the average value is determined. The integral is used as a measurement for the spread width. By scaling the deviations to the average value and forming the integral, a reliable criterion is also provided for judging the spread width.
According to another alternative, the measurement for the spread width is determined as a standard deviation of values of the output signal from an average value of the output signal. The standard deviation represents, as it were, a classical measurement for spread widths and can therefore also be used within the scope of the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.